Electrostatic discharge protection circuit and transistor

ABSTRACT

A circuit protects against electrostatic discharge and includes a transistor connected to the circuit to be protected. A semiconductor body of a first conductivity type serves as the collector. A first doped region of a second conductivity type is contained in the semiconductor body and serves as the base. A second doped region of the first conductivity type is contained in the first doped region and serves as the emitter. The first doped region includes a generally H-shaped doped region and a generally ring-shaped doped region forming an opening in which the second doped region serving as the emitter is received. The H-shaped doped region has a deeper junction surface than the junction surface of the ring-shaped doped region, and a dopant concentration that is less than the dopant concentration of the ring-shaped doped region. The H-shaped doped region achieves a low collector-to-base breakdown voltage and the ring-shaped doped region achieves a low snap-back voltage. A method for forming the transistor is also disclosed.

FIELD OF THE INVENTION

This invention relates to protecting semiconductor circuits againstelectrostatic discharge, and more particularly, to a protection circuithaving a transistor used for protecting semiconductor circuits againstelectrostatic discharge.

BACKGROUND OF THE INVENTION

Semiconductor integrated circuits are prone to electrostatic breakdownby static electricity charged on the human body or other similarsources. Such electrostatic discharge (ESD) causes breakdown of the PNjunction surface and breakdown of various films and layers formed on thedevice such as oxide films.

Various techniques have been devised for protecting circuits againstdamage caused by electrostatic discharge, such as those generated fromthe human body. These circuits have become very important because theslightest handling by an individual during semiconductor processing orin a final product could create enough electrostatic discharge to breakdown an oxide film or PN junction, thus ruining the semiconductordevice.

Some ESD protection devices and circuits use a combination of diodes andresistors to protect the semiconductor circuits. Other ESD protectioncircuits use various types of transistors, such as disclosed in U.S.Pat. No. 4,989,057 to Lu, which discloses a floating body field effecttransistor having a defined breakdown voltage, and a lower holdingvoltage to serve as a clamp for electrostatic discharge voltages,minimizing thermal power dissipation within the semiconductor layer.

U.S. Pat. No. 5,623,387 to Li, et al., uses a split bipolar transistorwith the transistor layout exhibiting very high tolerance to ESD events.The split bipolar transistor divides current among many segments andprevents current hogging, causing an ESD failure. The transistor usesthe snap-back effect to increase current carrying capacity. Thetransistor layout has metal contacts that are positioned away fromregions of high energy dissipation The use of high-diffusing phosphorousin various N-type doped regions prevents sharp changes in electrondensity.

Other circuits do not use as many contacts and doped regions, but makeuse of a transistor with the emitter and base shorted to show a bistablebehavior having a high impedance in the avalanche breakdown region and alow impedance in the bipolar snap-back region.

In such transistor devices, it is desirable to increase the clampingefficiency (i.e., limiting the voltage at a point in a circuit) bylimiting the maximum voltage developed at the collector terminal. It isalso desirable to have the smallest snap-back voltage to increase thedevice failure threshold during electrostatic discharge conditions.

These two requirements are typically mutually exclusive because a lowcollector-to-base breakdown voltage requires a generally low base dopantlevel, while a low clamping voltage during snap-back operation requiresa high current gain and a generally high base dopant level.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a circuitfor protecting against electrostatic discharge by increasing theclamping efficiency of protection and reducing the snap-back voltage.

It is also an object of the present invention to provide a transistorused in a circuit for protecting against electrostatic discharge thatachieves a low collector-to-base breakdown voltage and a low clampingvoltage during snap-back operation.

The circuit of the present invention now protects against electrostaticdischarge by using a transistor having a base with two doped regionsgiving a unique configuration so that one doped region achieves a lowcollector-to-base breakdown voltage and the other doped region achievesa low snap-back voltage. The circuit includes a pad, which receives asignal from an external signal source, such as a power source or othersignal. A semiconductor body of a first conductivity type serves as thecollector of the transistor and is connected to the pad. A first dopedregion of a second conductivity type is contained in the semiconductorbody and serves as the base of the transistor and forms acollector-to-base junction with the semiconductor body. A second dopedregion of the first conductivity type is contained in the first dopedregion and serves as the emitter of the transistor and forms abase-to-emitter junction with the first doped region.

Connection means electrically connects the first and second dopedregions and establishes a circuit connection between the base andemitter. The first doped region serving as the base further comprises agenerally H-shaped region and a generally ring-shaped doped regionforming an opening in which the second doped region serving as theemitter is received. The H-shaped doped region has a deeper junctiondepth than the ring-shaped doped region and a dopant concentration thatis less than the dopant concentration of the ring-shaped doped region sothat the H-shaped doped region achieves a low collector-to-basebreakdown voltage and the ring-shaped doped region achieves a lowsnap-back voltage.

In one aspect of the present invention, the H-shaped doped regionfurther comprises means for receiving and substantially encircling thering-shaped doped region. The receiving means defines a ring-shapedrecess that receives the ring-shaped doped region. The H-shaped dopedregion further comprises an inward formed medial section that exposes aportion of said ring-shaped doped region to the semiconductor bodyserving as a collector. The opening in the ring-shaped doped regionexposes a portion of the H-shaped doped region to the second dopedregion of first conductivity serving as the emitter. The H-shaped dopedregion further comprises a central slot and includes a doped region ofsecond conductivity received in the central slot and engaging theshorted connection means.

In still another aspect of the present invention, the shorted connectionmeans comprises a contact engaging both first and second doped regionsserving as the base and emitter. The shorted connection means can alsoinclude a ground connection. The semiconductor body serving as thecollector of the transistor further comprises an epitaxial layer. Thesemiconductor body also further comprises a sink for electrons.

In still another aspect of the present invention, the semiconductor bodyof the first conductivity type comprises an N-type material, and thedoped region of second conductivity type comprises a P-type material.

In a method aspect of the invention, a transistor is formed forprotecting against electrostatic discharge. A semiconductor body of afirst conductivity is formed to serve as the collector of thetransistor. A first doped region of a second conductivity is then formedin the semiconductor body to serve as the base of the transistor andform a collector-to-base junction with semiconductor body A second dopedregion of the first conductivity type is formed in the first dopedregion to serve as the emitter of the transistor and form abase-to-emitter junction with the first doped region and secondconductivity type.

The first doped region is further formed by forming a generally H-shapeddoped region and then forming a generally ring-shaped doped region andforming an opening in which the second doped region serving as theemitter is received. This H-shaped doped region is formed with a deeperjunction surface than said ring-shaped doped region and a dopantconcentration that is less than the dopant concentration of thering-shaped doped region so that when the first and second doped regionsare connected, the H-shaped doped region achieves a low collector-to-base breakdown voltage, and the ring-shaped doped region achieves a lowsnap-back voltage when the transistor is placed in the circuit subjectto electrostatic discharge.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the detailed description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIG. 1 is a schematic diagram of the protection circuit of the presentinvention;

FIG. 2 is a characteristic curve for the bipolar transistor of thepresent invention;

FIG. 3 is a top plan view of the transistor used in the protectioncircuit of the present invention;

FIG. 4 is an exaggerated sectional view of the transistor of FIG. 3taken along line 4--4; and

FIG. 5 is an isometric view of the H-shaped doped region of the gate,showing the region where the ring-shaped doped region is received andsubstantially encircled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now provides an electrostatic discharge (ESD)protection circuit, illustrated by the dotted line configuration at 10in FIG. 1, which protects another circuit, such as another semiconductorcircuit 12, against an electrostatic discharge occurring through contactwith human beings or other sources of static electricity. Typically, thecircuit 10 is formed on a single semiconductor substrate material as isknown to those skilled in the art. The present invention uses a uniquebipolar transistor illustrated generally at 14 having a collector, baseand emitter, 16, 18, 20. The collector 16 is formed from a doped regionof a first conductivity. The base 18 and emitter 20 are connectedtogether. The base 18 comprises a unique first doped region of secondconductivity having a generally H-shaped doped region and a generallyring-shaped doped region forming an open area to receive a second dopedregion of the first conductivity serving as the emitter. The H-shapeddoped region has a deeper junction depth than the ring-shaped dopedregion and a dopant concentration that is less than the dopantconcentration of the ring-shaped doped region so that the H-shaped dopedregion achieves a low collector-to- base breakdown voltage, and thering-shaped doped region achieves a low snap-back voltage.

FIG. 2 illustrates a characteristic curve of the transistor used in thecircuit 10 of the present invention circuit for protecting againstelectrostatic discharge. The base line on the curve is labeled Vc,indicating the voltage between the collector and the emitter. Thevertical line is labeled Ic, and indicates the generated current of thecollector 16 versus the applied voltage. The characteristic curveindicates that at a voltage point labeled A, corresponding to theavalanche breakdown point, the current across the collector increaseswhile the voltage also slightly increases until point B is reached. Atthis point, the voltage then snaps back to the snap back point locatedat point C. At this time, the transistor 14 exhibits very low resistanceand the current increases with only a slight increase in voltage asshown by the elevated line labeled D.

Referring once again to FIG. 1, the ESD circuit protection 10 of thepresent invention is illustrated in the schematic diagram. A pad 22 isconnected to an external signal source 23, which could be a power sourceor other signal source,. The collector 16 of the transistor 14 isconnected to the pad 22 and the base 18 and emitter 20 of the transistor14 are connected to each other and connected to the ground 24.

Referring now to FIGS. 3 and 4, greater details of the transistor 14 andcircuit 10 are shown. A semiconductor body of a first conductivity typeis generally shown at 30, and serves as the collector 16 of thetransistor 14. Typically, it is formed as part of a larger semiconductorsubstrate or similar body where many semiconductor components are formedas known to those skilled in the art. In one aspect of the presentinvention, the semiconductor body 30 of the first conductivity type isformed from an N-type material and includes a sink portion 32 and a bulkportion 34. As shown in the exaggerated sectional view of FIG. 4, wherehorizontal distances are exaggerated to show relation among varioustransistor 14 components, a collector contact 36 is positioned at thetop portion of the semiconductor body 30 and serves as the electricalcontact point, which can be connected to the pad 22. The sink portion 32supplies the charge carriers (holes or electrons) to the device asconventionally known to those skilled in the art.

An epitaxial layer 38 is formed on the semiconductor body 30 (orsubstrate) and has a lattice structure generally similar to that of thesemiconductor body and formed by techniques known to those skilled inthe art such as vapor phase epitaxial layer.

A first doped region, indicated generally at 40, of a secondconductivity type is contained in the semiconductor body 30 and servesas the base 18 of the transistor and forms a collector-to-base junction42 with the semiconductor body. In the illustrated embodiment, thisfirst doped region 40 is formed with a P-type impurity, forming a NPNbipolar transistor. It should be understood as known to those skilled inthe art that the transistor can also be formed as a PNP transistor. Inthat instance, the semiconductor body 30 would be doped with a P-typematerial. A second doped region 44 of the first conductivity type(N-type doped region) is contained in the first doped region 40 andserves as the emitter 20 of the transistor 14 and forms abase-to-emitter junction 46 with the first doped region 40. Shortedconnection means connect together the base 18 and emitter 20 formed byfirst and second doped regions 40, 44, and as illustrated in FIG. 3, areconnected together by a single contact 48, which can be connected toground 24.

As shown in FIGS. 3, 4 and 5, the first doped region of secondconductivity type includes an H-shaped doped region 50 (shown by angledsection lines sloping downward to the left), which includes aring-shaped recess 52 that receives a ring-shaped doped region 54 (shownby angled section lines sloping downward to the right). As shown in FIG.4, the H-shaped doped region 50 has a deeper junction 42 depth than thedepth of the ring-shaped doped region. The dopant concentration of theH-shaped doped region is also less than the dopant concentration of thering-shaped doped region 52. The H-shaped doped region further comprisesan inward formed medial section 60 forming the cross-piece of an H,which exposes a portion of the ring-shaped doped region 54 to thesemiconductor body serving as the collector 16. The medial section 60intersects the recess so that a side edge 62 of the ring-shaped dopedregion is exposed. This exposed portion of the ring-shaped doped regionis illustrated in FIG. 4. FIG. 3 illustrates a plan view showing thebasic configuration of the H-shaped doped region, its recess 52, and thering-shaped doped region received in recess 52.

The second doped region 44 of the first conductivity type is containedin the first doped region 40 and serves as the emitter of the transistor14 and forms the base-to-emitter junction 46 with the first doped region40. This second doped region 44 serving as the emitter 20 is alsogenerally ring-shaped and received within the area formed by the opening56. This opening is generally oval configured and defined in itsinterior by the dashed line 64 in FIG. 3 and exposes a portion of theH-shaped doped region underlying this area to this second doped region44 serving as the emitter 20. The ring-shaped emitter also forms anopening 65. As better shown in FIG. 5, the H-shaped doped regionincludes a central slot 66 formed in the ring-shaped recess 52 andincludes an upper doped region 67 (shown by dashed, downward slopinglines) of second conductivity (P-type material) received in the centralslot and engaging the pad to form an upper part of the transistor base18. This P-type material is formed as a generally elongate layer ofmaterial that is received within the central slot 66 of the H-shapeddoped region 50 and exposed to the contact 48 through the oval opening65 formed by the ring-shaped second doped region of the emitter. Thedotted line 64a in FIG. 3 corresponds to the periphery of the upperportion of the base formed by the upper doped region 67 having theP-type material engaging the emitter and contact.

Because of these two differences in the dopant concentration, thebreakdown region of the transistor is confined within the ring-shapeddoped region 54 that is not covered by the H-shaped doped region, i.e.,that portion such as the side edge 62, which is exposed to the collector16. This area forms a planar region whose junction is fairly deep andrugged to the electrostatic discharge event. Because of the high dopantconcentration of this ring-shaped doped region, the breakdown voltage ofthe base/collector junction 42 is low.

In the snap-back operation, the transistor action is mostly vertical.The opening 56 formed in the ring-shaped doped region confines thetransistor effect within that H-shaped doped region 50 of the base 18not covered (i.e., exposed) by the ring-shaped doped region, and thus,conductive with the emitter. This structure enhances the transistorcurrent gain and lowers the snap-back voltage of the formed transistor.Thus, if the failure threshold of the transistor is associated withelectrical power, which is dissipated within the snap-back region, thecircuit of the present invention greatly enhances the failure thresholdof the transistor at no detriment to clamping efficiency. The circuitallows a much lower snap-back voltage than it would be possible toachieve with the conventional, low breakdown base doping profile. Thecombined layout of the two doping profiles of the base is, therefore,advantageous.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is to be understood that the invention is not to belimited to the specific embodiments disclosed and that the modificationsand embodiments are intended to be included within the scope of theintended claims.

That which is claimed is:
 1. A circuit for protecting againstelectrostatic discharge comprising:a pad connected to a circuit to beprotected against electrostatic discharge; a semiconductor body of afirst conductivity type and serving as the collector of a transistor andconnected to said pad; a first doped region of a second conductivitytype contained in the semiconductor body and serving as the base of thetransistor and forming a collector-to-base junction with thesemiconductor body; a second doped region of the first conductivity typecontained in the first doped region and serving as the emitter of thetransistor and forming a base-to-emitter junction with the first dopedregion; connection means electrically connecting the first and seconddoped regions for establishing a connection between the base andemitter; said first doped region further comprising:a generally H-shapeddoped region, and a generally ring-shaped doped region forming anopening in which the second doped region serving as the emitter isreceived, wherein said H-shaped doped region has a deeper junction depththan said ring-shaped doped region and a dopant concentration of thesecond conductivity type that is less than the dopant concentration ofthe second conductivity type of the ring-shaped doped region, whereinsaid H-shaped doped region further comprises an inward formed medialsection that exposes a portion of said ring-shaped doped region formedas a side edge to said semiconductor body serving as the collector toform a planar region having a junction that is deep and rugged to anelectrostatic discharge event wherein the H-shaped doped region achievesa low collector-to-base breakdown voltage and the ring-shaped dopedregion achieves a low snap back voltage.
 2. The circuit according toclaim 1 wherein said connection means comprises a contact engaging boththe first and second doped regions serving as the base and emitter. 3.The circuit according to claim 1 wherein said H-shaped doped regionfurther comprises means for receiving and substantially encircling saidring-shaped doped region.
 4. The circuit according to claim 3 whereinsaid means for receiving said ring-shaped doped region further comprisesa recess that receives the ring-shaped doped region.
 5. The circuitaccording to claim 1 wherein said opening in said ring-shaped dopedregion exposes a portion of said H-shaped doped region to said seconddoped region of first conductivity serving as the emitter.
 6. Thecircuit according to claim 1 wherein said H-shaped doped region furthercomprises means defining a central slot, and including a doped region ofsecond conductivity received in the central slot and engaging saidconnection means.
 7. The circuit according to claim 1 wherein saidsemiconductor body serving as the collector of the transistor furthercomprises an epitaxial layer.
 8. The circuit according to claim 1wherein said semiconductor body serving as said collector furthercomprises a sink for electrons.
 9. The circuit according to claim 1wherein said connection means further comprises a ground connection. 10.The circuit according to claim 1 wherein said semiconductor body of afirst conductivity type comprises an n-type material, and said dopedregion of said second conductivity type comprises a p-type material. 11.A circuit for protecting against electrostatic discharge comprising:apad connected to a circuit to be protected against electrostaticdischarge; a semiconductor body of a first conductivity type and servingas the collector of a transistor and connected to said pad; a firstdoped region of a second conductivity type contained in thesemiconductor body and serving as the base of the transistor and forminga collector-to-base junction with the semiconductor body; a second,generally ring-shaped, doped region of the first conductivity typecontained in the first doped region and serving as the emitter of thetransistor and forming a base-to-emitter junction with the first dopedregion; connection means electrically connecting the first doped regionsfor establishing a connection between the base and emitter; said firstdoped region further comprising:a generally H-shaped doped region, and agenerally ring-shaped doped region forming an opening in which thesecond doped region serving as the emitter is received, wherein saidH-shaped doped region further comprises means defining a generallyring-shaped recess that receives and substantially encircles saidring-shaped doped region, and an inward formed medial section thatexposes a portion formed as a side edge of said ring-shaped doped regionto said semiconductor body serving as said collector to form a planarregion having a junction that is deep and rugged to an electrostaticdischarge event, and wherein said H-shaped doped section has a deeperjunction depth than said ring-shaped doped region and a dopantconcentration of the second conductivity type that is less than thedopant concentration of the second conductivity type of the ring-shapeddoped region whereby the first H-shaped doped region achieves a lowcollector-to-base breakdown voltage and the second ring-shaped dopedregion achieves a low snap back voltage and the opening formed in thering-shaped doped region confines the transistor effect.
 12. The circuitaccording to claim 11 wherein said connection means comprises a contactengaging both first and second doped regions serving as the base andemitter.
 13. The circuit according to claim 11 wherein said opening insaid ring-shaped doped region exposes a portion of said H-shaped dopedregion to said second doped region serving as the emitter.
 14. Thecircuit according to claim 13 wherein the H-shaped doped region exposedby said opening further comprises means defining a central slot, andincluding a doped region of second conductivity received in the centralslot and engaging said connection means.
 15. The circuit according toclaim 11 wherein said semiconductor body serving as the collector of thetransistor further comprises an epitaxial layer.
 16. The circuitaccording to claim 11 wherein said semiconductor body serving as saidcollector further comprises a sink for electrons.
 17. The circuitaccording to claim 11 wherein said connection means further comprises aground connection.
 18. The circuit according to claim 11 wherein saidsemiconductor body of a first conductivity type comprises a n-typematerial, and said first doped region comprises a p-type material.
 19. Atransistor for protecting against electrostatic discharge comprising:asemiconductor body of a first conductivity type and serving as thecollector of the transistor; a first doped region of a secondconductivity type contained in the semiconductor body and serving as thebase of the transistor and forming a collector-to-base junction with thesemiconductor body; a second doped region of the first conductivity typecontained in the first doped region and serving as the emitter of thetransistor and forming a base-to-emitter junction with the first dopedregion of second conductivity type; said first doped region furthercomprising:a generally H-shaped doped region, and a generallyring-shaped doped region forming an opening in which the second dopedregion serving as the emitter is received, wherein said H-shaped dopedregion has a deeper junction depth than said ring-shaped doped regionand a dopant concentration of the second conductivity type that is lessthan the dopant concentration of the second conductivity type of thering-shaped doped region, wherein said H-shaped doped region furthercomprises an inward formed medial section that exposes a portion of saidring-shaped doped region formed as a side edge to said semiconductorbody serving as the collector to form a planar region having a junctionthat is deep and rugged to an electrostatic discharge event wherein whenthe first and second doped regions are shorted, the H-shaped dopedregion achieves a low collector-to-base breakdown voltage and thering-shaped doped region achieves a low snap back voltage when thetransistor is placed in a circuit subject to electrostatic discharge.20. The transistor according to claim 19 including connection means forelectrically connecting both first and second doped regions serving asthe base and emitter.
 21. The transistor according to claim 19 whereinsaid H-shaped doped region further comprises means for receiving andsubstantially encircling said ring-shaped doped region.
 22. Thetransistor according to claim 21 wherein said means for receiving andencircling said ring-shaped doped region further comprises a recess thatreceives the ring-shaped doped region.
 23. The transistor according toclaim 19 wherein said opening in said ring-shaped doped region exposes aportion of said H-shaped doped region to said second doped regionserving as the emitter.
 24. The transistor according to claim 23 whereinsaid H-shaped doped region exposed by said opening further comprisesmeans defining a central slot, and including a doped region of secondconductivity received in the central slot and engaging said shortedconnection means.
 25. The transistor according to claim 19 wherein saidsemiconductor body serving as the collector of the transistor furthercomprises an epitaxial layer.
 26. The transistor according to claim 19wherein said semiconductor body serving as said collector furthercomprises a sink for electrons.
 27. The transistor according to claim 19wherein said shorted connection means further comprises a groundconnection.
 28. The transistor according to claim 19 wherein saidsemiconductor body of a first conductivity type comprises a n-typematerial, and said first doped region of second conductivity typecomprises a p-type material.